1. Field of the Invention
This invention relates to a power calculating device using a Hall element, and in particular relates to a power calculating device wherein imbalance voltage of the Hall element is automatically compensated.
2. Description of the Related Art
Power calculating devices of this kind are widely employed for wattmeters and for watthour meters on account of their simple construction and because the device lends itself to miniaturization.
Such a prior art power calculating device will now be described with reference to FIG. 7. The voltage of the system that is the subject of measurement, applied across terminals P1, P2, is input through a voltage converter 1 consisting of a voltage divider including resistors R1 and R2 or a transformer circuit to a voltage-current converter 2, where it is converted to a current A with the magnitude directly proportional to the input voltage. This current is applied to current terminals T1, T2 of a Hall element 3 as the control current of Hall element 3. Hall element 3 is equipped with output terminals T3, T4 arranged at right angles with respect to current terminals T1, T2. A magnetic field B is applied as shown in FIG. 7 in the direction at right angles to the direction of current terminals T1, T2 and to the direction of output terminals T3, T4 by means of a magnetic core 20 shown in FIG. 9. A Hall voltage V.sub.H of magnitude given by equation (1) directly proportional to the product of the magnetic flux density generated by the magnetic field and the control current flowing between current terminals T1 and T2 is generated across output terminals T3, T4 of Hall element 3. EQU V.sub.H =R . B . J (1)
where R is the Hall coefficient, B is the magnetic flux density, and J is the current density of the control current. A subtractor 4 is constructed with an operational amplifier OP1, and resistors R3, R4, R5 and R6. Subtractor 4 outputs a voltage proportional to the product of the control current and magnetic flux density by removing the in-phase component of Hall voltage V.sub.H generated across output terminals T3, T4. This voltage is inverted in polarity by inverting amplifier 5 as a voltage C, which is output outside as an output of power calculating device. FIG. 8 is a time-chart showing the operating waveform of various units of this power calculating device.
However, as the perfect symmetry of the output characteristic of Hall element 3 is not guaranteed, an offset voltage is generated due to an imbalance component D shown in FIG. 8. So that, there was the problem that the detection accuracy in output voltage C of the prior art power calculating device described above was lowered as shown in FIG. 8.